SQL2005 SP3 failed to install

There any many reasons that causes SP3 for SQL 2005 to fail and you can predict that by trying installing SQL2005 SP2 Security update.
One of the important reason to cause it fail: is removing the installer folder under windows or cleaning it ,so SQL Sp3 files will not continue.

This problem can be solved by extracting the SP3 files and copying all .MSP to c:\windows\installer. i tried and works without problems

Other problem related to Server SID change and this is full covered in http://support.microsoft.com/kb/925976

overall you have to check the log file and search for the reason that causes fail..